Magneto-optical recording media as rewritable high-capacity optical memory are the focus of attention, and magneto-optical disks have been utilized. Information is recorded on a magneto-optical recording medium by raising the temperature of the recording medium with the irradiation of laser light and applying an external magnetic field thereto. Meanwhile, the information is reproduced by irradiating laser light on the magneto-optical recording medium and detecting the rotating direction of the polarization plane of the reflected light.
Magneto-optical recording is roughly divided into two types: optical intensity modulation method and magnetic field modulation method. With the optical intensity modulation method, the intensity of laser light is modulated while keeping the external magnetic field having a uniform direction and strength. Here, as the direction of external magnetic field is uniform and bits are recorded along one direction, before recording the direction of magnetization needs to be opposite to the direction of recording. Meanwhile, with the magnetic field modulation method, the direction of external magnetic field is switched while keeping the laser light of a fixed intensity.
In this method, new information can be recorded while erasing previously recorded information. Therefore, the erasing of the previously recorded information is unnecessary, in practice shortening the time taken for recording. Accordingly, a lot of researches on this method have been carried out in recent years.
When the direction of magnetic field is switched, the magnetic field strength once becomes "0", thereby deteriorating the strength momentary. Thus, an area having information recorded with this low magnetic field does not have a sufficiently uniform magnetization direction. Therefore, reproduced signals from these areas have deteriorated quality. To restrain this, the period during which magnetic field has a low strength needs to be as short as possible and the magnetization direction needs to be switched as quick as possible.
However, normally external magnetic field generating means is made of an electromagnet composed of a coil wound around a ferrite core, and the magnetization direction is switched in accordance with the direction of a current flowing through the coil. Besides, a current having high frequencies hardly goes through the coil due to the coil's self-inductance. Therefore, in order to switch the magnetization direction at a high speed, a high voltage needs to be generated at both ends of the coil, preventing the manufacture of more compact device consuming less power.